The Nuts & Bolts of Burn-In & Reliability of PCB Assemblies

If you're producing systems for high-reliability applications, you need to know about burn-in. Here's how it impacts your product reliability and performance.

Burn in to weed out weaklings

If an electronic product is going to fail, it is most likely to do so soon after it is first put into use. A unit that makes it through this "Early Failure Period" is much more likely to last through its entire expected lifetime, until it wears out. This common chart of failure rate vs. time is widely known as "the Bathtub Curve."

Burn-in gets products through their Early Failure Period at the factory. Units that fail are rejected. Units that ship have entered their Intrinsic Failure Period. The end user sees the lower, intrinsic failure rate.

How do we do burn-in?

Burn-in involves powering up products and letting them run, then turning them off, and repeating. The power cycling creates the stress of inrush current and thermal expansion.

It's not practical to run each unit through its typical usage pattern over its entire Early Failure Period. This could take weeks or months... even years for some rarely-on but critical devices such as alarms.

Increasing the temperature condenses the early failure period. A rule of thumb is that the failure rate approximately doubles for every 10°C temperature increase. So at 55 to 60 °C (130 to 140 °F) we expect products to fail about eight times faster than at room temperature.

Why do products have an early failure period?

Sometimes early failure is caused by a random defective component or solder joint. Sometimes an entire batch of components is defective, causing higher-than-expected burn-in failure rates. Halting production to identify the issue before using more of the bad batch, we avoid the cost of board rework – or worse, a product recall.

Sometimes burn-in reveals a component change. For example, integrated circuit (IC) manufacturers love "die shrinks" to get more chips per wafer. The new IC has the same package, part number, appearance and specifications as the old one. We shouldn't notice anything different. However, there can be surprises.

When one of our products suddenly showed higher-than-expected early failure rates during burn-in, our engineers isolated the problem to a specific IC and contacted the manufacturer. We learned the IC had recently undergone a die shrink. With further testing we determined that the new, smaller chip was more susceptible to transient voltages. Our design engineers revised our schematic and board design to compensate.

Burn-in to stabilize components

Components change over time. Sometimes the amount of change is problematic, even if it's within specification. For example, the color of light is critical to the accuracy of a spectroscopy device.

Unfortunately a white LED changes color significantly in the first 24 hours of use. An LED-based blood analyzer, used a few times a day, could take months to stabilize. Burning in the system for 24 hours before shipping stabilizes the LEDs, for consistent results from the start.

Burn-in for design validation

When we put a new product into production, we start long-term burn-in on a number of the first units. This long-term burn-in simulates the entire expected lifetime of the product -- not just the Early Failure Period. In a year of burn-in we can simulate a decade or more of use.

If we see high failure rates after three months – equivalent to 2 1/2 years of actual use -- our engineers can investigate and make changes before manufacturing is fully ramped up.

How to talk about burn-in

Usually the product design engineer specifies the burn-in time and conditions. If you are buying design services, ask about the engineers' plan for long-turn burn-in to validate the new design.

If you are evaluating an EMS partner, check their ability to create burn-in fixtures and implement a burn-in plan for your products. The cost of burn-in is generally built into the manufacturing cost, although you may see a fee for the burn-in fixture as a non-recurring engineering charge.

The more costly a failure in the field, the more important it is to talk to your design and EMS partner about burn-in. Get a more detailed overview in the video below.

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